Carbon fiber reinforced composites are known for their outstanding mechanical properties associated with a low density. Some of those outstanding mechanical properties include superior tensile, flexural, and shear properties and impact resistance. For this reason, they have been of interest to many fields, particularly for rugged applications, such as the space and aeronautics industries, military equipment, transportation, and infrastructure.
Carbon fiber-epoxy composites are particularly used in such rugged applications. Although there has been a desire to extend the application of carbon fiber-epoxy composites to more commonplace markets, such as the automotive industry, tools, appliances, and sporting and recreational goods, their extension into these other markets has been substantially impeded by the higher cost of high performance epoxy resins relative to other resin systems. Less costly substitutes of epoxy resin have been sought, but the mechanical properties of these substitutes have thus far not approached the outstanding mechanical properties provided by high performance epoxy resins.
Vinyl ester resins are less costly than high performance epoxy resins, and are widely used, particularly because of their high resistance to moisture absorption and corrosion. Thus, vinyl ester resins would be a highly desirable substitute for an epoxy resin if only the resulting carbon fiber-vinyl ester resin composite could approach the outstanding mechanical properties provided by epoxy resin-based composites. However, the mechanical properties of carbon fiber-vinyl ester composites cannot currently compete with the mechanical properties of carbon fiber-epoxy composites. For this reason, carbon fiber-vinyl ester resin composites have not been considered for applications in which outstanding mechanical properties (e.g., high strength and ruggedness) are required.
The physico-chemical and mechanical properties of a composite material are not only dependent on the characteristics of the reinforcement material and the matrix, but also on the properties of the interface, which generally also depend on the conditions used during manufacturing. Some of the factors influencing interfacial adhesion include mechanical interlocking, physical and chemical interactions, the presence of defects, and residual thermal cure stress. In order to improve interfacial adhesion between the surface of a carbon fiber and a matrix, the properties of the carbon fiber surface can be modified by employing different types of surface treatments on the carbon fiber. Although some research has been conducted on carbon fiber surface treatment adapted to vinyl ester resin-based composites, the research is relatively limited and the mechanical properties that are obtained remain highly deficient in comparison to the properties provided by epoxy based systems.
Most of the techniques considered thus far for improving physical properties of vinyl ester composites aim at improving interfacial adhesion between the carbon fibers and the vinyl ester matrix by using the same approach used for epoxy-based systems, i.e., creating a covalent bonding between the surface of the carbon fiber and the matrix. For example, an epoxy sizing (carbon fiber coating) is typically partially cured during the composite manufacturing process. Indeed, the curing agent added to the epoxy matrix diffuses from the matrix to the sizing. However, when the same epoxy sizing is used on carbon fiber to make vinyl ester and polyester composites, the curing agent of the matrix (radical initiator) is not compatible with the polymerization of the epoxy sizing and is simply not effective in generating a suitable or optimal interface/interphase between the fibers and the matrix. Moreover, current technologies also do not take into account a specific property of vinyl ester resins (as well as of polyester resins), which is their high cure volume shrinkage. The cure volume shrinkage is typically 7% and higher for vinyl ester resins and up to 11% for polyester resins, in comparison with the 3-4% cure volume shrinkage experienced with epoxy resins.
Attempts at adjusting the surface properties of the carbon fibers using sizing agents have been made in an effort to counteract this adverse shrinkage phenomenon. Such efforts generally rely on mixing epoxy resins with a stoichiometric amount of curing agent (i.e., the curing degree of the epoxy reaches a value between 0.9 and 1). However, when a stoichiometric amount of curing agent is used in a sizing on carbon fiber, it results in an overactive curing state in which the cure of the epoxy sizing agent occurs at room temperature and continues until complete if left in ambient conditions for too long, e.g., over a day or more. Thus, using existing methodologies, it becomes necessary to control the time between the sizing of the fibers and the manufacture of the vinyl ester composite, in order to have components of the vinyl ester matrix (which commonly includes styrene) in contact with the epoxy sizing when its curing degree (i.e., fraction conversion or curing fraction) is optimal, such as 0.5, when the carbon fiber is more flexible. With longer curing times, the value of the curing degree continues to rise, resulting in a hard sizing that leads to rigid fibers, and rigid fibers are not suitable for composite processes. For this reason, the sizing or the sized fibers cannot be stored, which substantially obviates the implementation of this approach in most industrial applications. Moreover, the initiator of the vinyl ester matrix (e.g., peroxide) can react with amines, which are typically used as the curing agent for epoxy resins (and the epoxy sizing in this approach). The curing of the vinyl ester matrix and the epoxy sizing can be affected in the interdiffusion zone if these two curing agents are in contact and react with each other.